This invention relates to a thermal transfer recording medium for recording in such a manner that thermally fusible ink is transferred to a transfer material by using a thermal head.
A thermal transfer recording method has been widely used as a method of easily recording onto a plain paper (transfer paper). However, the quality of the printed characters can easily depend upon the smoothness of the surface of the transfer paper. It is, therefore, difficult to clearly print characters on the transfer paper which is inferior in surface smoothness. In order to overcome this problem, the following methods have previously been proposed: a method in which heat treatment is carried out after printing has been completed (Japanese Patent Application Laid-Open (KOKAI) No. 58-76276); a method in which a supplementary means such as a masgnetic force (Japanese Patent Application Laid-Open (KOKAI) No. 52-96549) or static electrical force (Japanese Patent Application Laid-Open (KOKAI) No. 55-65590) is used when the transfer is performed; a method in which a great quantity of oily material is added into an ink so as to lower the melt viscosity of ink at the time of performing transfer (Japanese Patent Application Laid-Open (KOKAI) No. 60-25762); and a method in which a heat-decomposable material (Japanese Patent Application Laid-Open (KOKAI) No. 60-82389) or a thermally expandable material (Japanese Patent Application Laid-Open (KOKAI) No. 60-25762) is added to the ink so as to improve sensitivity to heat. Further, a method for improving the quality of printed characters by using a multilayered thermally fusible ink-layer has been proposed. For example, there are proposed a method in which thermally fusible inks faving slightly different fusing temperatures each other are laminated in the form of layers and pigment is added to at least one of the layers (Japanese Patent Application Laid-Open (KOKAI) No. 59-224392), a method in which a layer of a thermally fusible material which does not contain coloring material is formed on the thermally fusible ink layer (Japanese Patent Application Laid-Open (KOKAI) No. 60-97888).
However, in the case of the above-described recording methods in which ink having been fused to become a liquid are transferred onto the paper, the quality of the characters printed on the paper with poor surface smoothness is insufficient as compared with that of characters printed on the paper with high surface smoothness. For this reason, the problem involved in the thermal transfer recording method, that is to say, the defect that the quality of the printed characters depends upon the smoothness of the paper surface onto which the characters are to be transferred, cannot be satisfactorily dissolved.
On the other hand, high quality printed characters can be obtained on a paper sheet which is inferior in surface smoothness by using ink mainly composed of a resin showing a tackness and having a certain mechanical strength without becoming a liquid with a low viscosity when the thermal energy is applied thereto, so that the ink adheres to convex portions on the surface of the paper sheet and covers concave portions on the surface of the paper sheet.
However, since the use of such a resin ink described above requires a high energy as compared with conventional wax-type inks, in the case where the resin ink is used, a film exhibiting an excellent heat resistance needs as a supporting film and, in addition problems concerning the life of the thermal head and concerning heat accumulation arise.
It is known that a releasing layer is disposed under a resin ink layer for improving the sensitivity of the resin ink, such releasing layer being made of a material such as wax which can be easily fused.
A thermal transfer recording medium having such a structure is known as a bridge transfer-type recording medium. In bridge transfer, the critical factor is the timing of separating the thermal transfer recording medium from the transfer paper onto which characters are transferred by the application of energy for performing transfer. In the case where this timing is late and as a result, the releasing layer to be separated and the ink layer have cooled down, a problem thus arises that if the adherence of ink to the paper is weak, the ink returns to the thermal transfer recording medium, so that transfer can not be achieved. On the other hand, a further problem arises that the sharpness is impaired because the separation of the ink in the printed portion causes the separation of the ink located at other portions where no printing is to be performed (this phenomenon is referred to as "accompanied transfer").
A line printer which prints character lines one by one by using a line head exhibits high recording speed. However, the timing of separating the thermal transfer recording medium from the paper after application of energy for performing printing is late as compared with the timing in serial printers which prints characters one by one. Therefore, it is difficult to print by using a bridge-type thermal transfer recording medium which is designed for use in a serial printer.
As a result of the inventors' studies in order to solve the above-mentioned problems, it has been found that as an ink layer of a thermal transfer recording medium, by using an ink layer comprising:
a first thermally fusible material layer stacked on one surface of a heat resistant supporting material;
at least one second thermally fusible material layer stacked on a surface of the first thermally fusible material layer which is opposite to a surface of the first thermally fusible layer facing to the supporting material; and
a third thermally fusible material layer stacked on a surface of the second thermally fusible material layer which is opposite to a surface of the second thermally fusible layer facing to the supporting material,
wherein an elongation rate of the second thermally fusible material layer at 20.degree. C. is higher than the respective elongation rates of the first and third thermally fusible material layers at 20.degree. C.,
a clear and correct recording can be obtained on a transfer paper which is poor in surface smoothness. Based on this finding, the present invention has been attained.